1. Field of the Invention
The present invention relates to a method and apparatus for determining a status of a laser gas mixture within a discharge chamber of a gas discharge laser, and more particularly for measuring a parameter of the output laser beam versus the applied driving discharge voltage or other adjustable input parameter and comparing the measured data with a master set or sets of stored data to determine the status of the laser gas mixture and/or whether any electrical, mechanical or optical problems exist within the system.
2. Discussion of the Related Art
Gas discharge lasers, e.g., excimer or molecular lasers, are well known as valuable tools for many industrial applications. There is a great desire to have precise control and simultaneous stabilization of many laser parameters over extended durations of operation, especially with regard to excimer laser applications in many fields including electronics and photolithographic processing. The amount of "up time" of a laser, or time when a laser is in operation and being used for industrial application, is a key variable in operation cost considerations. It is desired to be able to successively adjust, sensitively control and carefully stabilize various laser parameters efficiently and simultaneously.
The type and quality of the gas discharge affects many significant laser parameters such as output power, energy stability, efficiency, bandwidth, long and short axial beam profiles, temporal and spatial pulse width, and beam divergence and coherence. The quality of the gas discharge depends on such factors as the composition of the gas mixture in the discharge chamber, the quality of preionization used, properties of the discharge circuit, and profiles of the electrodes used. See R. S. Taylor, Appl. Phys. B41, 1-24 (1986). Decomposition and contamination of the gas mixture and the design of the gas exchange system (e.g., flow speed) also strongly determine the limits of achievable laser parameters. A fast gas exchange between electrodes may be realized by using a laser discharge chamber design including fast blower gas circulation. Cryogenic and electrostatic equipment and techniques may be used for additional gas purification. See German Patent No. 32 12 928.
Optimum gas mixtures for various gas discharge lasers are generally known. A partition of 0.1:1.0:98.9 F.sub.2 :Kr:Ne is thought to be substantially optimal for a KrF-excimer laser, for example, and 0.1:99.9 F.sub.2 :Ne for an F.sub.2 laser. FIG. 1 shows a plot of laser output power versus F.sub.2 concentration and represents a way of determining what the optimal F.sub.2 concentration actually is. As time goes by and the laser is operated, the gas mixture degrades or "ages" continuously resulting in a dilution of F.sub.2 and a consumption of F.sub.2 via chemical reactions with metal dust. In this regard, U.S. Pat. No. 4,977,573 to Bittenson et al., which is assigned to the same assignee as the present application, is incorporated herein by reference. After a parameter changes by a certain amount, such as after a certain amount of time, number of pulses or change in discharge voltage compensating the gas deterioration to maintain constant output energy, among others, a replenishment such as halogen injection (HI) or partial gas replacement (PGR) of a certain amount of the gas mixture or an entirely new fill of the gas mixture is performed to, as nearly as possible, substantially reinstate the original partition of the gas and optimize laser parameters.
It is desired to be able to prolong the lifetime of the laser gas mixture. It is further desired to have suitable measuring tools that indicate when and to what extent the laser gas mixture is aged before problems associated with laser parameters varying from optimum as the laser gas degrades lead to significant reductions in laser output performance, processing errors and excessive laser downtime.
A mass spectrometer may be used for precision analysis of the composition of the gas mixture. See U.S. Pat. No. 5,090,020 to Bedwell. However, a mass spectrometer is an undesirably hefty and costly piece of equipment to incorporate into a continuously operating excimer or molecular laser system. Other ways of monitoring the status of a laser gas mixture include measuring a spectrum width or bandwidth of a laser emission (see U.S. Pat. No. 5,450,436 to Mizoguchi et al.), measuring a beam profile of the laser emission (see U.S. Pat. No. 5,642,374 to Wakabayashi et al.), and measuring other characteristics such as the width of the discharge or temporal pulse width of the output beam wherein a rough estimate of the status of the gas mixture may be made. See U.S. Pat. No. 5,440,578 to Sandstrom. Another known technique of measuring the age of the laser gas mixture is to count the total number of laser pulses from the most recent new fill of the discharge chamber. See U.S. Pat. No. 5,646,954 to Das et al.
A number of techniques are known wherein the output beam energy or efficiency is monitored and steps are taken to maintain the output beam at an optimum energy. See U.S. Pat. Nos. 3.899,750 to Hochuli, 4,429,392 to Yoshida et al., and 4,977,573 to Bittenson et al. Rare and halogen gas concentrations have also been maintained by using a complex series of chemical reactions to determine the gas mixture concentrations and replenish depleted gases as needed. See U.S. Pat. No. 4,740,982 to Hakuta et al.
The above parameters measured and monitored for determining the status of the laser gas mixture are each dependent on other parameters in addition to the gas mixture status, e.g., stabilized output energy, repetition rate, etc. They are based on generally known behaviors of laser systems and general experience regarding gas mixture aging in discharge chambers. It is desired to have a technique for monitoring the gas mixture status without variations in other parameters affecting the analysis. It is also desired that properties of the discharge chamber, optics and discharge circuit, among others, be taken into account in a gas mixture status monitoring procedure to provide greater completeness and accuracy.